• NAME
• DESCRIPTION
• Perl Functions by Category
• Portability
• Alphabetical Listing of Perl Functions

NAME

perlfunc - Perl builtin functions

DESCRIPTION

The functions in this section can serve as terms in an expression. They fall into two major categories: list operators and named unary operators. These differ in their precedence relationship with a following comma. (See the precedence table in the perlop manpage.) List operators take more than one argument, while unary operators can never take more than one argument. Thus, a comma terminates the argument of a unary operator, but merely separates the arguments of a list operator. A unary operator generally provides a scalar context to its argument, while a list operator may provide either scalar or list contexts for its arguments. If it does both, the scalar arguments will be first, and the list argument will follow. (Note that there can ever be only one such list argument.) For instance, splice() has three scalar arguments followed by a list, whereas gethostbyname() has four scalar arguments.

In the syntax descriptions that follow, list operators that expect a list (and provide list context for the elements of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Elements of the LIST should be separated by commas.

Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the parentheses.) If you use the parentheses, the simple (but occasionally surprising) rule is this: It looks like a function, therefore it is a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. And whitespace between the function and left parenthesis doesn't count--so you need to be careful sometimes:

    print 1+2+4;        # Prints 7.
    print(1+2) + 4;     # Prints 3.
    print (1+2)+4;      # Also prints 3!
    print +(1+2)+4;     # Prints 7.
    print ((1+2)+4);    # Prints 7.

If you run Perl with the -w switch it can warn you about this. For example, the third line above produces:

    print (...) interpreted as function at - line 1.
    Useless use of integer addition in void context at - line 1.

A few functions take no arguments at all, and therefore work as neither unary nor list operators. These include such functions as time and endpwent. For example, time+86_400 always means time() + 86_400.

For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in a scalar context by returning the undefined value, and in a list context by returning the null list.

Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value it would be most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in list context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.

An named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. You can't get a list like (1,2,3) into being in scalar context, because the compiler knows the context at compile time. It would generate the scalar comma operator there, not the list construction version of the comma. That means it was never a list to start with.

In general, functions in Perl that serve as wrappers for system calls of the same name (like chown(2), fork(2), closedir(2), etc.) all return true when they succeed and undef otherwise, as is usually mentioned in the descriptions below. This is different from the C interfaces, which return -1 on failure. Exceptions to this rule are wait, waitpid, and syscall. System calls also set the special $! variable on failure. Other functions do not, except accidentally.

Perl Functions by Category

Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some functions appear in more than one place.

Functions for SCALARs or strings

chomp, chop, chr, crypt, hex, index, lc, lcfirst, length, oct, ord, pack, q/STRING/, qq/STRING/, reverse, rindex, sprintf, substr, tr///, uc, ucfirst, y///

Regular expressions and pattern matching

m//, pos, quotemeta, s///, split, study, qr//

Numeric functions

abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, srand

Functions for real @ARRAYs

pop, push, shift, splice, unshift

Functions for list data

grep, join, map, qw/STRING/, reverse, sort, unpack

Functions for real %HASHes

delete, each, exists, keys, values

Input and output functions

binmode, close, closedir, dbmclose, dbmopen, die, eof, fileno, flock, format, getc, print, printf, read, readdir, rewinddir, seek, seekdir, select, syscall, sysread, sysseek, syswrite, tell, telldir, truncate, warn, write

Functions for fixed length data or records

pack, read, syscall, sysread, syswrite, unpack, vec

Functions for filehandles, files, or directories

-X, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link, lstat, mkdir, open, opendir, readlink, rename, rmdir, stat, symlink, umask, unlink, utime

Keywords related to the control flow of your perl program

caller, continue, die, do, dump, eval, exit, goto, last, next, redo, return, sub, wantarray

Keywords related to scoping

caller, import, local, my, package, use

Miscellaneous functions

defined, dump, eval, formline, local, my, reset, scalar, undef, wantarray

Functions for processes and process groups

alarm, exec, fork, getpgrp, getppid, getpriority, kill, pipe, qx/STRING/, setpgrp, setpriority, sleep, system, times, wait, waitpid

Keywords related to perl modules

do, import, no, package, require, use

Keywords related to classes and object-orientedness

bless, dbmclose, dbmopen, package, ref, tie, tied, untie, use

Low-level socket functions

accept, bind, connect, getpeername, getsockname, getsockopt, listen, recv, send, setsockopt, shutdown, socket, socketpair

System V interprocess communication functions

msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, shmctl, shmget, shmread, shmwrite

Fetching user and group info

endgrent, endhostent, endnetent, endpwent, getgrent, getgrgid, getgrnam, getlogin, getpwent, getpwnam, getpwuid, setgrent, setpwent

Fetching network info

endprotoent, endservent, gethostbyaddr, gethostbyname, gethostent, getnetbyaddr, getnetbyname, getnetent, getprotobyname, getprotobynumber, getprotoent, getservbyname, getservbyport, getservent, sethostent, setnetent, setprotoent, setservent

Time-related functions

gmtime, localtime, time, times

Functions new in perl5

abs, bless, chomp, chr, exists, formline, glob, import, lc, lcfirst, map, my, no, prototype, qx, qw, readline, readpipe, ref, sub*, sysopen, tie, tied, uc, ucfirst, untie, use

* - sub was a keyword in perl4, but in perl5 it is an operator, which can be used in expressions.

Functions obsoleted in perl5

dbmclose, dbmopen

Portability

Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the functionality of some Unix system calls may not be available, or details of the available functionality may differ slightly. The Perl functions affected by this are:

-X, binmode, chmod, chown, chroot, crypt, dbmclose, dbmopen, dump, endgrent, endhostent, endnetent, endprotoent, endpwent, endservent, exec, fcntl, flock, fork, getgrent, getgrgid, gethostent, getlogin, getnetbyaddr, getnetbyname, getnetent, getppid, getprgp, getpriority, getprotobynumber, getprotoent, getpwent, getpwnam, getpwuid, getservbyport, getservent, getsockopt, glob, ioctl, kill, link, lstat, msgctl, msgget, msgrcv, msgsnd, open, pipe, readlink, rename, select, semctl, semget, semop, setgrent, sethostent, setnetent, setpgrp, setpriority, setprotoent, setpwent, setservent, setsockopt, shmctl, shmget, shmread, shmwrite, socket, socketpair, stat, symlink, syscall, sysopen, system, times, truncate, umask, unlink, utime, wait, waitpid

For more information about the portability of these functions, see the perlport manpage and other available platform-specific documentation.

Alphabetical Listing of Perl Functions

-X FILEHANDLE

-X EXPR

-X

A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename or a filehandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests $_, except for -t, which tests STDIN. Unless otherwise documented, it returns 1 for true and '' for false, or the undefined value if the file doesn't exist. Despite the funny names, precedence is the same as any other named unary operator, and the argument may be parenthesized like any other unary operator. The operator may be any of:

    -r  File is readable by effective uid/gid.
    -w  File is writable by effective uid/gid.
    -x  File is executable by effective uid/gid.
    -o  File is owned by effective uid.

    -R  File is readable by real uid/gid.
    -W  File is writable by real uid/gid.
    -X  File is executable by real uid/gid.
    -O  File is owned by real uid.

    -e  File exists.
    -z  File has zero size.
    -s  File has nonzero size (returns size).

    -f  File is a plain file.
    -d  File is a directory.
    -l  File is a symbolic link.
    -p  File is a named pipe (FIFO), or Filehandle is a pipe.
    -S  File is a socket.
    -b  File is a block special file.
    -c  File is a character special file.
    -t  Filehandle is opened to a tty.

    -u  File has setuid bit set.
    -g  File has setgid bit set.
    -k  File has sticky bit set.

    -T  File is an ASCII text file.
    -B  File is a "binary" file (opposite of -T).

    -M  Age of file in days when script started.
    -A  Same for access time.
    -C  Same for inode change time.

Example:

    while (<>) {
        chop;
        next unless -f $_;      # ignore specials
        #...
    }

The interpretation of the file permission operators -r, -R, -w, -W, -x, and -X is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file. Such reasons may be for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats.

Also note that, for the superuser on the local filesystems, the -r, -R, -w, and -W tests always return 1, and -x and -X return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode of the file, or temporarily set their effective uid to something else.

If you are using ACLs, there is a pragma called filetest that may produce more accurate results than the bare stat() mode bits. When under the use filetest 'access' the above-mentioned filetests will test whether the permission can (not) be granted using the access() family of system calls. Also note that the -x and -X may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Read the documentation for the filetest pragma for more information.

Note that -s/a/b/ does not do a negated substitution. Saying -exp($foo) still works as expected, however--only single letters following a minus are interpreted as file tests.

The -T and -B switches work as follows. The first block or so of the file is examined for odd characters such as strange control codes or characters with the high bit set. If too many strange characters (>30%) are found, it's a -B file, otherwise it's a -T file. Also, any file containing null in the first block is considered a binary file. If -T or -B is used on a filehandle, the current stdio buffer is examined rather than the first block. Both -T and -B return true on a null file, or a file at EOF when testing a filehandle. Because you have to read a file to do the -T test, on most occasions you want to use a -f against the file first, as in next unless -f $file && -T $file.

If any of the file tests (or either the stat or lstat operators) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with -t, and you need to remember that lstat() and -l will leave values in the stat structure for the symbolic link, not the real file.) Example:

    print "Can do.\n" if -r $a || -w _ || -x _;

    stat($filename);
    print "Readable\n" if -r _;
    print "Writable\n" if -w _;
    print "Executable\n" if -x _;
    print "Setuid\n" if -u _;
    print "Setgid\n" if -g _;
    print "Sticky\n" if -k _;
    print "Text\n" if -T _;
    print "Binary\n" if -B _;

abs VALUE

abs

Returns the absolute value of its argument. If VALUE is omitted, uses $_.

accept NEWSOCKET,GENERICSOCKET

Accepts an incoming socket connect, just as the accept(2) system call does. Returns the packed address if it succeeded, false otherwise. See the example in Sockets: Client/Server Communication in the perlipc manpage.

On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in the perlvar manpage.

alarm SECONDS

alarm

Arranges to have a SIGALRM delivered to this process after the specified number of seconds have elapsed. If SECONDS is not specified, the value stored in $_ is used. (On some machines, unfortunately, the elapsed time may be up to one second less than you specified because of how seconds are counted.) Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer.

For delays of finer granularity than one second, you may use Perl's four-argument version of select() leaving the first three arguments undefined, or you might be able to use the syscall interface to access setitimer(2) if your system supports it. The Time::HiRes module from CPAN may also prove useful.

It is usually a mistake to intermix alarm and sleep calls. (sleep may be internally implemented in your system with alarm)

If you want to use alarm to time out a system call you need to use an eval/die pair. You can't rely on the alarm causing the system call to fail with $! set to EINTR because Perl sets up signal handlers to restart system calls on some systems. Using eval/die always works, modulo the caveats given in Signals in the perlipc manpage.

    eval {
        local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
        alarm $timeout;
        $nread = sysread SOCKET, $buffer, $size;
        alarm 0;
    };
    if ($@) {
        die unless $@ eq "alarm\n";   # propagate unexpected errors
        # timed out
    }
    else {
        # didn't
    }

atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the Math::Trig::tan function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }

bind SOCKET,NAME

Binds a network address to a socket, just as the bind system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in Sockets: Client/Server Communication in the perlipc manpage.

binmode FILEHANDLE, DISCIPLINE

binmode FILEHANDLE

Arranges for FILEHANDLE to be read or written in ``binary'' or ``text'' mode on systems where the run-time libraries distinguish between binary and text files. If FILEHANDLE is an expression, the value is taken as the name of the filehandle. DISCIPLINE can be either of ":raw" for binary mode or ":crlf" for ``text'' mode. If the DISCIPLINE is omitted, it defaults to ":raw".

binmode() should be called after open() but before any I/O is done on the filehandle.

On many systems binmode() currently has no effect, but in future, it will be extended to support user-defined input and output disciplines. On some systems binmode() is necessary when you're not working with a text file. For the sake of portability it is a good idea to always use it when appropriate, and to never use it when it isn't appropriate.

In other words: Regardless of platform, use binmode() on binary files, and do not use binmode() on text files.

The open pragma can be used to establish default disciplines. See the open manpage.

The operating system, device drivers, C libraries, and Perl run-time system all work together to let the programmer treat a single character (\n) as the line terminator, irrespective of the external representation. On many operating systems, the native text file representation matches the internal representation, but on some platforms the external representation of \n is made up of more than one character.

Mac OS and all variants of Unix use a single character to end each line in the external representation of text (even though that single character is not necessarily the same across these platforms). Consequently binmode() has no effect on these operating systems. In other systems like VMS, MS-DOS and the various flavors of MS-Windows your program sees a \n as a simple \cJ, but what's stored in text files are the two characters \cM\cJ. That means that, if you don't use binmode() on these systems, \cM\cJ sequences on disk will be converted to \n on input, and any \n in your program will be converted back to \cM\cJ on output. This is what you want for text files, but it can be disastrous for binary files.

Another consequence of using binmode() (on some systems) is that special end-of-file markers will be seen as part of the data stream. For systems from the Microsoft family this means that if your binary data contains \cZ, the I/O subsystem will ragard it as the end of the file, unless you use binmode().

binmode() is not only important for readline() and print() operations, but also when using read(), seek(), sysread(), syswrite() and tell() (see the perlport manpage for more details). See the $/ and $\ variables in the perlvar manpage for how to manually set your input and output line-termination sequences.

bless REF,CLASSNAME

bless REF

This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package. If CLASSNAME is omitted, the current package is used. Because a bless is often the last thing in a constructor, it returns the reference for convenience. Always use the two-argument version if the function doing the blessing might be inherited by a derived class. See the perltoot manpage and the perlobj manpage for more about the blessing (and blessings) of objects.

Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase names, so to prevent confusion, you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value.

See Perl Modules in the perlmod manpage.

caller EXPR

caller

Returns the context of the current subroutine call. In scalar context, returns the caller's package name if there is a caller, that is, if we're in a subroutine or eval or require, and the undefined value otherwise. In list context, returns

    ($package, $filename, $line) = caller;

With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.

    ($package, $filename, $line, $subroutine, $hasargs,
    $wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);

Here $subroutine may be (eval) if the frame is not a subroutine call, but an eval. In such a case additional elements $evaltext and $is_require are set: $is_require is true if the frame is created by a require or use statement, $evaltext contains the text of the eval EXPR statement. In particular, for a eval BLOCK statement, $filename is (eval), but $evaltext is undefined. (Note also that each use statement creates a require frame inside an eval EXPR) frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with. The $hints and $bitmask values are subject to change between versions of Perl, and are not meant for external use.

Furthermore, when called from within the DB package, caller returns more detailed information: it sets the list variable @DB::args to be the arguments with which the subroutine was invoked.

Be aware that the optimizer might have optimized call frames away before caller had a chance to get the information. That means that caller(N) might not return information about the call frame you expect it do, for N > 1. In particular, @DB::args might have information from the previous time caller was called.

chdir EXPR

Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the directory specified by $ENV{HOME}, if set; if not, changes to the directory specified by $ENV{LOGDIR}. If neither is set, chdir does nothing. It returns true upon success, false otherwise. See the example under die.

chmod LIST

Changes the permissions of a list of files. The first element of the list must be the numerical mode, which should probably be an octal number, and which definitely should not a string of octal digits: 0644 is okay, '0644' is not. Returns the number of files successfully changed. See also oct, if all you have is a string.

    $cnt = chmod 0755, 'foo', 'bar';
    chmod 0755, @executables;
    $mode = '0644'; chmod $mode, 'foo';      # !!! sets mode to
                                             # --w----r-T
    $mode = '0644'; chmod oct($mode), 'foo'; # this is better
    $mode = 0644;   chmod $mode, 'foo';      # this is best

You can also import the symbolic S_I* constants from the Fcntl module:

    use Fcntl ':mode';

    chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
    # This is identical to the chmod 0755 of the above example.

chomp VARIABLE

chomp LIST

chomp

This safer version of chop removes any trailing string that corresponds to the current value of $/ (also known as $INPUT_RECORD_SEPARATOR in the English module). It returns the total number of characters removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried that the final record may be missing its newline. When in paragraph mode ($/ = ""), it removes all trailing newlines from the string. When in slurp mode ($/ = undef) or fixed-length record mode ($/ is a reference to an integer or the like, see the perlvar manpage) chomp() won't remove anything. If VARIABLE is omitted, it chomps $_. Example:

    while (<>) {
        chomp;  # avoid \n on last field
        @array = split(/:/);
        # ...
    }

You can actually chomp anything that's an lvalue, including an assignment:

    chomp($cwd = `pwd`);
    chomp($answer = <STDIN>);

If you chomp a list, each element is chomped, and the total number of characters removed is returned.

chop VARIABLE

chop LIST

chop

Chops off the last character of a string and returns the character chopped. It's used primarily to remove the newline from the end of an input record, but is much more efficient than s/\n// because it neither scans nor copies the string. If VARIABLE is omitted, chops $_. Example:

    while (<>) {
        chop;   # avoid \n on last field
        @array = split(/:/);
        #...
    }

You can actually chop anything that's an lvalue, including an assignment:

    chop($cwd = `pwd`);
    chop($answer = <STDIN>);

If you chop a list, each element is chopped. Only the value of the last chop is returned.

Note that chop returns the last character. To return all but the last character, use substr($string, 0, -1).

chown LIST

Changes the owner (and group) of a list of files. The first two elements of the list must be the numeric uid and gid, in that order. A value of -1 in either position is interpreted by most systems to leave that value unchanged. Returns the number of files successfully changed.

    $cnt = chown $uid, $gid, 'foo', 'bar';
    chown $uid, $gid, @filenames;

Here's an example that looks up nonnumeric uids in the passwd file:

    print "User: ";
    chomp($user = <STDIN>);
    print "Files: ";
    chomp($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
        or die "$user not in passwd file";

    @ary = glob($pattern);      # expand filenames
    chown $uid, $gid, @ary;

On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption. On POSIX systems, you can detect this condition this way:

    use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
    $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);

chr NUMBER

chr

Returns the character represented by that NUMBER in the character set. For example, chr(65) is "A" in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face (but only within the scope of a use utf8). For the reverse, use ord. See the utf8 manpage for more about Unicode.

If NUMBER is omitted, uses $_.

chroot FILENAME

chroot

This function works like the system call by the same name: it makes the named directory the new root directory for all further pathnames that begin with a / by your process and all its children. (It doesn't change your current working directory, which is unaffected.) For security reasons, this call is restricted to the superuser. If FILENAME is omitted, does a chroot to $_.

close FILEHANDLE

close

Closes the file or pipe associated with the file handle, returning true only if stdio successfully flushes buffers and closes the system file descriptor. Closes the currently selected filehandle if the argument is omitted.

You don't have to close FILEHANDLE if you are immediately going to do another open on it, because open will close it for you. (See open.) However, an explicit close on an input file resets the line counter ($.), while the implicit close done by open does not.

If the file handle came from a piped open close will additionally return false if one of the other system calls involved fails or if the program exits with non-zero status. (If the only problem was that the program exited non-zero $! will be set to 0.) Closing a pipe also waits for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards, and implicitly puts the exit status value of that command into $?.

Prematurely closing the read end of a pipe (i.e. before the process writing to it at the other end has closed it) will result in a SIGPIPE being delivered to the writer. If the other end can't handle that, be sure to read all the data before closing the pipe.

Example:

    open(OUTPUT, '|sort >foo')  # pipe to sort
        or die "Can't start sort: $!";
    #...                        # print stuff to output
    close OUTPUT                # wait for sort to finish
        or warn $! ? "Error closing sort pipe: $!"
                   : "Exit status $? from sort";
    open(INPUT, 'foo')          # get sort's results
        or die "Can't open 'foo' for input: $!";

FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the real filehandle name.

closedir DIRHANDLE

Closes a directory opened by opendir and returns the success of that system call.

DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle name.

connect SOCKET,NAME

Attempts to connect to a remote socket, just as the connect system call does. Returns true if it succeeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See the examples in Sockets: Client/Server Communication in the perlipc manpage.

continue BLOCK

Actually a flow control statement rather than a function. If there is a continue BLOCK attached to a BLOCK (typically in a while or foreach), it is always executed just before the conditional is about to be evaluated again, just like the third part of a for loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the next statement (which is similar to the C continue statement).

last, next, or redo may appear within a continue block. last and redo will behave as if they had been executed within the main block. So will next, but since it will execute a continue block, it may be more entertaining.

    while (EXPR) {
        ### redo always comes here
        do_something;
    } continue {
        ### next always comes here
        do_something_else;
        # then back the top to re-check EXPR
    }
    ### last always comes here

Omitting the continue section is semantically equivalent to using an empty one, logically enough. In that case, next goes directly back to check the condition at the top of the loop.

cos EXPR

cos

Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes cosine of $_.

For the inverse cosine operation, you may use the Math::Trig::acos() function, or use this relation:

    sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }

crypt PLAINTEXT,SALT

Encrypts a string exactly like the crypt(3) function in the C library (assuming that you actually have a version there that has not been extirpated as a potential munition). This can prove useful for checking the password file for lousy passwords, amongst other things. Only the guys wearing white hats should do this.

Note that crypt is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) corresponding decrypt function. As a result, this function isn't all that useful for cryptography. (For that, see your nearby CPAN mirror.)

When verifying an existing encrypted string you should use the encrypted text as the salt (like crypt($plain, $crypted) eq $crypted). This allows your code to work with the standard crypt and with more exotic implementations. When choosing a new salt create a random two character string whose characters come from the set [./0-9A-Za-z] (like join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]).

Here's an example that makes sure that whoever runs this program knows their own password:

    $pwd = (getpwuid($<))[1];

    system "stty -echo";
    print "Password: ";
    chomp($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $pwd) ne $pwd) {
        die "Sorry...\n";
    } else {
        print "ok\n";
    }

Of course, typing in your own password to whoever asks you for it is unwise.

The crypt function is unsuitable for encrypting large quantities of data, not least of all because you can't get the information back. Look at the by-module/Crypt and by-module/PGP directories on your favorite CPAN mirror for a slew of potentially useful modules.

dbmclose HASH

[This function has been largely superseded by the untie function.]

Breaks the binding between a DBM file and a hash.

dbmopen HASH,DBNAME,MASK

[This function has been largely superseded by the tie function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal open, the first argument is not a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If the database does not exist, it is created with protection specified by MASK (as modified by the umask). If your system supports only the older DBM functions, you may perform only one dbmopen in your program. In older versions of Perl, if your system had neither DBM nor ndbm, calling dbmopen produced a fatal error; it now falls back to sdbm(3).

If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an eval, which will trap the error.

Note that functions such as keys and values may return huge lists when used on large DBM files. You may prefer to use the each function to iterate over large DBM files. Example:

    # print out history file offsets
    dbmopen(%HIST,'/usr/lib/news/history',0666);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    dbmclose(%HIST);

See also the AnyDBM_File manpage for a more general description of the pros and cons of the various dbm approaches, as well as the DB_File manpage for a particularly rich implementation.

You can control which DBM library you use by loading that library before you call dbmopen():

    use DB_File;
    dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
        or die "Can't open netscape history file: $!";

defined EXPR

defined

Returns a Boolean value telling whether EXPR has a value other than the undefined value undef. If EXPR is not present, $_ will be checked.

Many operations return undef to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish undef from other values. (A simple Boolean test will not distinguish among undef, zero, the empty string, and "0", which are all equally false.) Note that since undef is a valid scalar, its presence doesn't necessarily indicate an exceptional condition: pop returns undef when its argument is an empty array, or when the element to return happens to be undef.

You may also use defined(&func) to check whether subroutine &func has ever been defined. The return value is unaffected by any forward declarations of &foo.

Use of defined on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate has ever been allocated. This behavior may disappear in future versions of Perl. You should instead use a simple test for size:

    if (@an_array) { print "has array elements\n" }
    if (%a_hash)   { print "has hash members\n"   }

When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use exists for the latter purpose.

Examples:

    print if defined $switch{'D'};
    print "$val\n" while defined($val = pop(@ary));
    die "Can't readlink $sym: $!"
        unless defined($value = readlink $sym);
    sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
    $debugging = 0 unless defined $debugging;

Note: Many folks tend to overuse defined, and then are surprised to discover that the number